1. Field of the Invention
The present invention relates to a surface-area light source device that is applicable to liquid crystal display devices represented by television monitors, personal computer monitors, and the like. The present invention also relates to a liquid crystal display device having such a surface-area light source device.
2. Description of the Related Art
Today, flat liquid crystal display devices having a backlight mechanism are widely used as monitors for television sets and personal computers.
A liquid crystal display device having the mechanism mentioned above includes a liquid crystal display panel formed of two glass plates and a liquid crystal layer sandwiched between the two glass plates, and a surface-area light source device placed on the rear surface side of the liquid crystal display panel. The liquid crystal display panel transmits or blocks light from the surface-area light source device, and thereby an image is displayed.
The surface-area light source device includes a light guide plate and a light source disposed on a side surface thereof; light emitted from the light source enters the light guide plate from the side surface to be smoothed in the shape of a plane by being repeatedly reflected and refracted inside the light guide plate, and is uniformly outputted from a top surface of the light guide plate. Cold-cathode tubes have conventionally been used as the light source described above, but recently, light emitting diodes, which are brighter and have a longer life than cold-cathode tubes, have come to be used as well.
In the case where a light emitting diode, which is a dot light source, is used in a surface-area light source device of a large liquid crystal display device such as a television monitor and a personal computer monitor, a large number of light emitting diodes need to be provided in order to secure a sufficient amount of light.
However, in the case where a thin light guide plate is used for the purpose of achieving a thinner and lighter surface-area light source device and thereby achieving a thinner and lighter liquid crystal display device, the light guide plate does not have a wide side surface, and thus the space for light emitting diodes is limited. This may make it impossible to provide a sufficient number of light emitting diodes, resulting in an insufficient amount of light of the surface-area light source device. In addition, some arrangements of light emitting diodes may prevent light from being uniformly outputted from the light guide plate, causing unevenness in brightness and color.
To solve these problems, Japanese Patent Laid-open No. 2004-342472 (pages 3 to 5, FIG. 3) proposes a surface-area light source device in which light is uniformly outputted from a light guide plate. This surface-area light source device is provided with dot light sources arranged in two rows near a side surface of a light guide plate and along a long side of the side surface of the light guide plate. The dot light sources of a second row are arranged such that they do not overlap the dot light sources of a first row, and thereby dark spaces between any two light sources are eliminated to allow light to uniformly enter the light guide plate, and as a result, uniform emission of light from the light guide plate is achieved.
However, with the surface-area light source device proposed in Japanese Patent Laid-open No. 2004-342472, since light emitting-diode chips serving as the light source are arranged in two rows on a side surface, if the distance between the two rows is small, adjacent ones of the light emitting diodes give and receive heat they generate to and from each other. As a result, the temperature of the light emitting diodes rises to cause the light emission intensity and the emission wavelength of the light emitting diodes to change, which results in a non-uniform emission of light from the light guide plate. On the other hand, if a smaller amount of current is supplied to the light emitting diodes to reduce the amount of heat they generate, light from the light guide plate deteriorates in brightness.